<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var year = prompt('请输入你的出生年份：');
        var age = 2021 -year;
        alert('你今年'+ age +'岁了')
    </script>
</head>
<body>
    
</body>
</html>